I just went to the Physical Therapist.
AC joint issue. Impingement. That the acromial process of the scapula/ shoulderblade that meets up with the clavicle to hold the humerus/ upper arm bone in place. Largely, one of the several "rotator cuff" muscles is weak, and tight pectoral muscles means my shoulders are forward and the humerus isn't held tight in the right place against the A and C, which was resulting is grinding and pain.
Water, stretching, external rotation exercise, a low row exercise, and something else with a theraband. Doing external rotations while lying on my side in bed without weight, to build endurance.
Stand tall, shoulders back, head high, chest out, tight core muscles. Stretch. Hydrate. Rinse and repeat.